1. Field of the Invention
This invention relates to an absolute position inductive transducer. More particularly, this invention is directed to an absolute position inductive transducer that includes a compact, long-range code track for extending the length of the absolute position scale.
2. Description of Related Art
U.S. Pat. No. 4,893,077 to Auchterlonie describes an absolute position sensor employing several linear tracks of inductive transducers. Each track of this sensor has a slightly different wavelength or frequency. The circuits in the sensor analyze the phase difference between the tracks to determine the absolute position of the read head. Similar known systems employ capacitive transducers having multiple tracks of capacitive elements, such as U.S. Pat. Nos. 4,879,508 and 5,023,599 to Andermo. The absolute position sensors of Auchterlonie and Andermo, however, suffer from a number of problems, including scale length limitations, sensitivity to contamination, increased manufacturing costs due to tight tolerance requirements, and difficulty to incorporate into hand-held devices.
U.S. Pat. No. 4,697,144 to Howbrook discloses a transducer that employs several pitches of coils (each pitch representing 360.degree. of phase change) to similarly provide an absolute position using an inactive member. This transducer, however, has a limited range within which to determine the absolute position of the inactive member. Additionally, this transducer fails to provide sufficient accuracy for most applications.
U.S. Pat. No. 5,027,526 to Crane describes an optical transducer that reads a bar code pattern printed on a coiled tape. This bar code pattern is the standard interleaved 2 of 5 bar code symbol that encodes several numbers between start and stop bar code patterns. The numbers, in turn, correspond to a coarse absolute position of the tape. Circuits read the bar code symbols and convert them to numbers representing the absolute position of the tape. Clockings based on the position of a drum that coils the tape determine a fine position measurement.
This absolute transducer, however, suffers from traditional problems of optical transducers, such as scale length limitations, sensitivity to contamination, increased manufacturing costs, and large current supply requirements. Furthermore, this absolute transducer is not a true absolute transducer at every position, because the transducer requires a scanning motion through a range as long as the bar code in order to derive or update an absolute position measurement. This renders it unusable for many applications.
U.S. patent application Ser. Nos. 08/788,469 now U.S. Pat. No. 5,886,519 and 08/790,494 now U.S. Pat. No. 5,841,274 to Masreliez et al., filed Jan. 29, 1997, each herein incorporated by reference in its entirety, disclose a number of longer-range absolute position transducers. One current absolute position transducer increases the absolute position range by using multiple analog tracks with different repeat lengths. However, the current state of the art for inductive and capacitive transducers imposes a maximum practical ratio of wavelengths between tracks of about 32:1 (regardless of whether the ratio is established by the primary wavelengths, or by a beat frequency between closely spaced wavelengths), a minimum for the fine wavelengths from 1.28 mm to 5.12 mm, and read head lengths of at least five fine wavelengths for most metrology applications. Longer fine wavelengths provide proportionately lower resolution and accuracy. Therefore, the maximum length of a two-track scale would be 32 fine wavelengths (about 40 to 160 mm). Longer scales would require more tracks, more read heads, and wider overall scale width, thus are more expensive and require a larger physical size. A typical scale with a fine wavelength of 2.56 mm would require 3 tracks and read heads to achieve a scale length of between 80 mm and 2500 mm.
Another current absolute position transducer disclosed in Masreliez uses binary coded tracks to increase the absolute position range. This transducer requires a code track having N-bit code words and N read heads to achieve a coarse scale length of 2.sup.N fine wavelengths. A scale would require 8 read heads to achieve a coarse scale length of 256 fine wavelengths. This transducer uses a pseudo-random sequence of code words analyzed along the code track. Shifting the read head by one code position anywhere in the sequence will generate a unique code word, distinguishable from all other code words. Each code word position corresponds to and uniquely identifies a particular fine wavelength of a fine wavelength scale having approximately 2.sup.N fine wavelengths. Once the particular fine wavelength is identified, the fine wavelength scale can be used to identify the absolute position to a fine resolution. However, the length of this transducer is limited to the length of the coarse wavelength. Further, not all code words are usable because of the inability to unambiguously determine certain code words.
Another current transducer uses separator marks such as start, stop, and parity bits between code words. Therefore, in a binary system which can read an 8-bit code word and uses three bits to accomplish synchronization, the maximum scale length would be 8*2.sup.(8-3) (256) bit positions.
Yet another current transducer uses continuously varying wavelengths. This technique will work with a single track. However, the fine scale accuracy degrades as the wavelength increases toward the ends of the scale, since there are fewer fine scale marks under the read head. Additionally, the reduced spacing between the marks decreases the contrast between the phases. In this device, the scale length is limited by the read head length, the minimum spacing which allows marks to be accurately distinguished, and the minimum number of marks which are required under the read head for adequate accuracy.